Effects of externally applied tensile stresses on the moisture diffusion characteristics of epoxy glass composites

Abstract

Marine Hydrokinetic (MHK) Power involves using the power of moving water to create clean, renewable energy. The primary structure of MHK energy devices are most commonly constructed using epoxy glass composite materials. Unstressed epoxy glass composites absorb moisture when subjected to a humid environment; this moisture absorption leads to degradation of mechanical properties of the composite. This phenomenon is relatively well documented and understood. However, under most operating conditions the structure will be under some combination of externally applied stresses. The objective of this study is to characterize the effects of externally applied stresses on the moisture diffusion parameters of epoxy glass laminates, and how these changes ultimately influence mechanical properties. A model is proposed which relates externally applied tensile stresses to changes in absorption capacity as well as diffusion rate. The model postulates that changes seen in the diffusion process are the result of stress-dependent changes in the free volume of the epoxy resin. The free volume changes of the resin are calculated through laminate plate theory; the free volume change becomes a function of fiber angle as well as a host of elastic properties of the constituents. Consequently, according to the proposed model, changes in diffusion parameters are dependent upon the magnitude of applied stress, the loading angle, as well as elastic properties of the constituents. In order to experimentally test the proposed model, a series of epoxy glass laminate samples were manufactured at varying fiber angles and submerged in a moist environment while subjected to varying levels of tensile loading. Weight gain measurements we recorded throughout the diffusion process until full saturation was achieved. The experimental values exhibited excellent agreement with the novel theoretical model.